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I feel wet cells as the power source might be easier to attain than a generator. There are several different designs, some based on two metals, others on a metal and carbon, and then the electrolyte, sometimes alcalic, sometimes acidic. Wet cells will wear out and need replacing, but their manufacture doesn't need quite as much mechanical prowess and infrastructure as a generator.

For good decent generator, ferromagnetic metal is unavoidable. I don't, however, know the properties of natural magnets sufficiently well to give a definitive ruling on this. For a self-excited generator, only a small permanent magnet field is required. But the windings should really be supported by magnetically permeable metal.

For the electrical circuitry, almost any conductive material will do. It is only a matter of losses. Tin, brass, bronze, lead, silver, gold...

I was thinking of the vacuum. That's going to be your stumbling stone. In a vacuum tube there is a hard vacuum; even the industrial manufacturers don't get it good enough. Instead, after having exhausted their mechanical pumps, diffusion and ion traps, they seal the envelope and fire a small getter charge to trap the residual gases into solid compounds. But to get to that point, they're using technolgy that is pretty challenging to replicate.

I think I am still advocating the spark gap transmitter.